Pyrolytic carbon may be deposited by thermally decomposing gaseous hydrocarbons or other carbonaceous substances in vaporous form in the presence of a substrate whereupon the deposition will take place. It is well known to coat substrates with layers of pyrolytic carbon for various different purposes. In this respect, the coating may oftentimes completely envelop the substrate, and the composite coated substrate may be the desired end product. In other instances, a disposable object or mandrel may be used as the substrate and be coated with a fairly thick layer of pyrolytic carbon; subsequently, the mandrel is machined away or otherwise removed whereby the monolithic coating becomes the desired end product, e.g., European Patent No. 0 055 406, inventors: Bokros et al., incorporated herein by reference. The present invention is suitable for use with all such instances, even if the underlying substrate is eventually removed.
When pyrolytic carbon is deposited in a fluidized bed apparatus, one of the variables upon which the structure of the pyrolytic carbon will be dependent is the available deposition surface area, relative to the volume of the furnace enclosure wherein the deposition is occurring. Pyrolytic carbon which has a microstructure that is free of growth features will be deposited when the relative amount of deposition surface area is fairly high. Thus, when relatively large objects, for example, objects having at least one dimension equal to 5 mm or more, are being coated, an ancillary bed of small particles (usually of a size measured in hundreds of microns) are included within the furnace enclosure together with the larger object or objects. This arrangement provides sufficient available total surface area to assure that pyrolytic carbon having the desired crystalline form will be deposited. In addition, the random motion of large objects in fluidized beds provide for a relatively uniform deposition of carbon on all surfaces.
However, whenever such submillimeter particles are being coated in a fluidized bed, the total surface area of the particles begins to increase significantly as the diameters of the pyrolytic carbon-coated particles grow. This change in the available deposition surface area in the fluidized bed will result in a change in the physical characteristics of the pyrolytic carbon being deposited if the other coating variables are held constant, e.g., coating temperature, gas flow rate and gas composition; and moreover, when the bed reaches some maximum size, it will collapse and thus limit the thickness of the carbon coating that can be deposited on levitated substrates under constant conditions. Changes in the physical characteristics of the carbon deposited may be undesirable for any of a number of reasons.
It has been found that pyrolytic carbon having good structural strength and uniform physical properties can be deposited as relatively thick coatings upon relatively large objects in the accompaniment of particles if the available fluidized bed surface area is maintained relatively constant by withdrawing particles which have become enlarged in size as a result of coating and feeding smaller size particles into the deposition enclosure. Very generally, the availability of a relatively large amount of deposition surface area in a furnace enclosure of a given volume facilitates the efficient deposition of pyrolytic carbon which is either isotropic or laminar in microstructure and without growth features. In contrast when carbon is deposited on a fixed substrate (e.g., a mandrel) in a chamber without a bed of particles, large gradients in gas composition are established at the gas-solid interfaces where the deposition is occurring, and growth features develop in the microstructure of the deposited carbon. Illustrations and theoretical considerations are reviewed in J. C. Bokros, "The Preparation, Structure, and Properties of Pyrolytic Carbon," in Chemistry and Physics of Carbon, Vol. 5, P. L. Walker (ed.) Marcel Dekker, New York, 1969, Chapter 1.
The crystalline structure, the density and other physical properties, such as the coefficient of thermal expansion, of pyrolytic carbon deposited by the thermal decomposition of a vaporous carbonaceous substance are dependent upon several independently variable operating conditions within the coating apparatus being employed. These conditions include the temperature of the substrate surfaces upon which the deposition is occurring, the overall chemical composition of the atmosphere from which deposition is occurring, the partial pressure of the vaporous carbonaceous substance, the surface area to volume ratio in the active deposition region of the coating apparatus, and the contact time (a parameter based upon the gas flow rate and cross sectional area of the furnace enclosure). Although various of these conditions can be easily regulated and therefore maintained at a constant desired value in many different types of coating apparatus, the surface area to volume ratio is inherently subject to constant change because there is a continuous gradual increase in the total surface area as the items being coated grow in size as the result of the deposition thereupon. When a bed of small spheroids or the like, having an average size between about 50 microns and 600 microns, is present in the active deposition surface region (either because they are the products being coated, e.g., in the case of nuclear fuel particles, or because they are associated with other objects being coated so as to increase the total surface area to void volume ratio), such small particles increase relatively rapidly in surface area as the diameters of these particles grow during deposition of pyrolytic carbon.
A desired surface area to volume ratio is initially provided by starting with an appropriate amount of particles of a particular average size to constitute the fluidized bed. Preferably, an initial surface area to volume ratio is provided near the lower end of the range that produces crystalline pyrolytic carbon having the physical properties desired. Thereafter, as the growing thickness of the deposited pyrolytic carbon layers causes the total surface area to increase, withdrawal of some of the coated particles is initiated so as to decrease the total number of particles to thereby maintain a specific surface area or to regulate its increase in a controlled manner, and replacement of the particles being withdrawn with particles of much smaller size is also begun.
In coating operations where it is desirable to employ a relatively large surface area to volume ratio, a coating apparatus is of course employed that can maintain such a bed of particles in motion and in association with any larger objects that may also be coated. Examples of suitable coating apparatus of this type include, fluidized bed coaters and rotating drum coaters.
One example of a suitable coating apparatus is shown in Bokros, et al. U.S. Pat. No. 3,399,969 incorporated herein by reference, which points out that the pyrocarbon coating of relatively large objects, such as objects having a dimension of about 5 millimeters or greater, in the presence of an ancillary bed of small particles (i.e. of a size measured in tens or hundreds of microns), is best controlled by controlling the available deposition surface area relative to the volume of the coating enclosure wherein the actual deposition is taking place.
In Bokros and Akins U.S. Pat. No. 3,977,896 and Akins, incorporated herein by reference, an improved process was described and illustrated for depositing pyrolytic carbon coatings of substantial thickness which would have a very uniform crystallinity throughout the entire thickness of the pyrocarbon deposited. Such uniformity was achieved by maintaining the total deposition surface area within the coating enclosure relatively constant by adding particles of small size to the coater while coated particles (which have grown in size) are being removed from the enclosure at a controlled rate by regulating the rate of flow of an inert gas stream upward through a conduit through which the particles exit.
Subsequent to the issuance of the '896 patent, Brooks U.S. Pat. No. 4,546,012 incorporated herein by reference, which teaches an improved fluidized bed apparatus for coating such a relatively large object in association with a bed of particles being levitated, which maintains a constant bed size from a volumetric standpoint by employing a weir tube having a spillover entrance hole to define the maximum upper level of the fluidized bed by its location at a desired vertical level within the coating enclosure. Preferably the weir tube is closed at its top and has its spillover hole facing away from the centerline of the coating enclosure. Sufficient purge flow of inert gas upward through the tube is maintained so as to prevent any substantial quantity of dust from gravitating down the tube with the coated particles being withdrawn. Brooks U.S. Pat. No. 4,594,270 incorporated herein by reference, and shows an apparatus for removing particles from a desired level within such a fluidized bed within which a larger object is being levitated and coated by pyrolytic decomposition. This patent teaches the employment of a vertically slidable sampling tube, preferably controlled from the exterior of the furnace, which can be located so as to have its opening at a desired vertical level from which the sample is to be withdrawn. Flow of inert gas through the sampling tube is controlled so as to selectively allow particles to be withdrawn from the bed when the tube is at the desired vertical level.
Although the foregoing patents disclose acceptable processes for controlling pyrolytic deposition within a fluidized bed system, it is extremely difficult, if not impossible, to determine the actual size of the bed surface area at any moment, and as coating processes become more sophisticated, there is a desire to be able to even more precisely control the characteristics of the pyrocarbon being deposited. As a result, more improved methods of control have been sought.